Method and apparatus for detecting submarines

ABSTRACT

A method for detecting, tracking and locating submarines ( 24 ) utilizes pulsed coherent radiation from a laser ( 12 ) that is projected down through a water column, with particles in the water producing speckle from backscatter of the random particle distribution, with correlation of two closely time-spaced particle-based speckle patterns providing an intensity measurement indicative of the presence of a submarine. Subsurface submarine movement provides a subsurface wake which causes movement of particles such that two closely-spaced “snapshots” of the returns from particles in the same water column can detect particle movement due to the wake. The magnitude of the speckle pattern change indicates particle movement. In one embodiment, the return signals are imaged onto an intensified CCD or APA array that capture two successive laser pulses through the utilization of dual pixel registered cameras. Note that in the subject system, phase information is converted to measurable intensity information relating to particle motion.

CROSS REFERENCE TO RELATED APPLICATION

This application claims rights under 35 USC § 119(e) from U.S.application Ser. No. 60/501,149 filed Sep. 5, 2003.

FIELD OF THE INVENTION

This invention relates to submarine detection and more particularly to amethod for detecting sub-sea particle motion due to wake turbulenceproduced by a submarine.

BACKGROUND OF THE INVENTION

Detection and tracking of submarines has long been the goal ofantisubmarine warfare. In the past, sonobuoys and the like have beenused to sense the presence of submerged submarines through theirdeployment and the use of sophisticated sonar techniques. Additionally,worldwide hydrophone arrays have been utilized in an attempt to locateand track submarines.

However, with passive acoustic techniques, the development of noiselessor silent submarines oftentimes makes it impossible to detect such quietsubmarines due to their quiet-running design. Moreover, installing ordeploying hydrophone arrays to detect the presence of submarines is bothcostly and time-consuming, thus significantly limiting the search area.

With respect to magnetometers, in order to detect a magnetic anomaly,one must first have a magnetic map of the terrain against which tocompare the magnetometer readings from an overlying aircraft. Thedevelopment of these maps with sufficient accuracy has been problematicin the past and the isolation of the magnetometer from the aircraftitself, usually on a boom, presents it own problems.

As described in U.S. Pat. No. 4,893,294, a different system fordetecting the presence of submarines by Donald A. Leonard and Harold E.Sweeney involves the sensing of thermoclines which are produced bysub-sea wake turbulence when a submarine moves through the water. Asdescribed in the Encyclopedia of Oceanography, Vol. 1, Rhodes PairbridgeEdition, pp. 402-408, Rhineholt Publishing Corporation of New York,1996, subsurface waves are produced by the movement of a body throughthe water which creates so-called internal waves. The above patentdescribes a way to measure the internal waves by constructing a profileof ocean water temperatures as a function of depth. While techniqueshave been employed in the past for measuring these profiles involvingthe use of many temperature sensors, these are costly and time-consumingto deploy, making them unsuited to high spatial and temporal resolutioncoverage of large ocean areas.

In an effort to improve on the early techniques for measuring thethermoclines, thus to detect submarines by measurement of sub-surfaceocean temperatures, the above-noted patent describes a system for remotetemperature sensing by using a laser carried by an attack or searchsubmarine. In this patent the remote temperature sensing device uses apulsed laser to illuminate an area of interest which includes thethermocline region. Part of the temperature sensing device is an opticalreceiver which picks up the laser backscatter and uses theself-heterodyne of the wavelength-shifted Brillouin scatter with theunshifted Rayleigh scatter.

According to this patent the self-heterodyne action allows mixing of theabove signals from each volume element of the illuminated water columnindependently by measuring the frequency of the heterodyne signal. Sincethe frequency is directly related to the water temperature and sincetime is directly proportional to depth, the resultant time-temperaturepattern is said to be equivalent to a temperature test profile. Movementof a submarine through the water creates internal waves that affect thisprofile such that the above-noted technique is said to provide along-persisting indication of movement of a submerged vessel.

It will be noted that the laser is a sub-sea laser which is to bemounted on a submarine that executes a search pattern in various sub-sealocations. This is inherently a slow process because of the relativelylow top speed of a subsurface vessel. Secondly, the technique describedin the above-mentioned patent detects a long-persisting temperaturechange and does not effectively locate the position of the submarinethat is detected. Temperature changes persist over long periods of timesuch that the temperature changes engendered by the passage of asubmarine can exist, for instance, over 20 miles, hardly a techniquesuitable for localization and tracking of a submarine. Thirdly, thesystem operates by detecting frequency shifts which are correlated totemperature changes. Thus, while the above technique may be able todetect wake, its ability to localize and track a submarine isquestionable.

SUMMARY OF INVENTION

On the other hand, subsurface vehicles such as submarines when movingthrough the water can be detected in the subject invention by thedetection of the movement of particles in the wake of the submarine.While the vast majority of the ocean has particles which do not move butfor Brownian motion, which has a long period, when a turbulence isgenerated by a submarine, the particles in the wake of the submarinemove violently. It is the movement of the particles that is detected inthe subject invention, not temperature variation. In one embodiment adouble snapshot technique is used in which a water column is illuminatedby an overflying laser which projects a beam down normal to the surfaceof the ocean to illuminate the water column. Two laser pulses separatedby between 10 and 100 microseconds produce returns which are separatelydetected on an array, with each of the returns producing its own specklepattern. The speckle patterns, which are spaced apart in time by tens ofmicroseconds, are subtracted one from the other in a cross-correlationtechnique, with the intensity of the cross-correlation detectingparticle motion and thus the presence of a submarine wake. How thespeckle patterns are formed is now described.

In the subject method, rays of backscattered light interfere amongthemselves. Hence, the system may be referred to as autodyne, and thesystem is not affected by uniform changes in ray pathlength. Since allinterfering rays pass through the same medium, much larger displacementscan be tolerated, and a moving air/water interface will not degradedetection performance. This self-referencing property makes the systemsensitive to differential motion between scatterers and insensitive toother motion, which is a very critical characteristic for sensingunderwater turbulence due to motion of the platform, sea surface, andintervening water.

Surface reflections at the air/sea interface are eliminated by rangegating techniques. Moreover, the same range gating techniques are usedto determine the depth of the sub-sea wake, such that only a range-gatedvolume of water is interrogated. The range gating thus determines thedepth of the wake, with the downwardly-projected laser beam determiningthe position of the particles moving as a result of the passage of thesubmarine. Since the motions produced by the wake turbulence of asub-sea vessel have a short period as compared to the period of Brownianmotion, the subject technique is effective in detecting subsurfacevehicle-produced turbulence. This is because for undisturbed ocean theparticles move very little or not at all. In short, they are stationary.

Moreover, since the particle motion dies out relatively quickly ascompared to thermocline temperature change reduction, the location ofthe subsurface vessel can be tracked to a greater degree than heretoforepossible.

In one embodiment a double-pulse laser mounted on an overflying aircraftprojects pulses spaced by 10 to 100 microseconds into the water. Thepulsed coherent illumination of the water column produces backscatterfrom random particle distribution, which in turn produces speckle on adetector array. Cross-correlation of closely time-spaced specklepatterns permits the derivation of an intensity measurement that denotesparticle movement. The sensor in one embodiment is range gated for depthdiscrimination so that only the range gated portion of the water columnis read out.

The returns are imaged on an intensified CCD or APD array, with the twosuccessive laser pulses captured by either a single fast readout or by adual pixel registered detector array. Outputs from the double snapshotdetector array enable the determination of the magnitude of specklepattern change. The change is manifested as speckle pattern timevariation, where the water motion causes the change in the specklereturn. The phases of the resulting intensity variations are due torelative particle motion from the object moving through the water, withphase information converted to measurable intensity informationindicative of particle motion.

In summary, a method for detecting, tracking and locating submarinesutilizes pulsed coherent radiation from a laser is projected downthrough a water column, with particles in the water producing specklefrom backscatter of the random particle distribution, with correlationof two closely time-spaced particle-based speckle patterns providing anintensity measurement indicative of the presence of a submarine.Subsurface submarine movement provides a subsurface wake which causesmovement of particles such that two closely-spaced “snapshots” ofreturns from particles in the same water column can detect particlemovement due to the wake. The magnitude of the speckle pattern changeindicates particle movement. In one embodiment, the return signals areimaged onto an intensified CCD or APA array that capture two successivelaser pulses through the utilization of dual pixel registered cameras.Note that in the subject system, phase information is converted tomeasurable intensity information relating to particulate motion.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the subject invention will be betterunderstood in connection with a Detailed Description, in conjunctionwith the Drawings, of which:

FIG. 1 is a diagrammatic illustration of the subject system illustratingthe utilization of a double pulse laser illuminating a water column,with moving particles providing reflected energy in the form of aspeckle pattern detected at a detector, with sequential speckle patternsbeing correlated to provide an intensity correlation indicating particlemovement engendered by a submarine wake;

FIG. 2 is a diagrammatic illustration of the movement of particleswithin a water column due to the wake of a submerged vessel; and,

FIG. 3 is a diagrammatic illustration of the generation of sequentialspeckle patterns in a range gated segment of the water columncorresponding to a predetermined depth.

DETAILED DESCRIPTION

Referring to FIG. 1, in one embodiment a submarine detection system 10includes a double pulse Nd:YAG laser 12 having its output redirected bya half-silvered mirror 14 and through optics 16 so as to illuminate thesurface 18 of the ocean. Note that the second harmonic of the Nd:YAGlaser is used for its green light.

The laser beam enters the water orthogonal to the water surface anddefines a water column 20 extending from the surface of the ocean downthrough underwater waves generally indicated by 20 caused by the wake 22of a submarine 24.

Double pulse laser 12 is controlled by a timing unit 26, which initiatesthe double pulse laser train 28 in which the pulse peaks are separatedby between 10 and 100 microseconds. Note that in a search pattern thedouble pulse laser is fired repeatedly at the same area of the ocean tobe able to detect returns from various ocean depths by range gating.

The initiation of the pulses from timing unit 26 is used in a variabledelay circuit 28 to be able to range gate out reflections from thesurface of the ocean and to specify where in the water column a rangegated segment lies. Thus, the round trip travel time of the laser beamdown into the water where it is reflected by particles and returnedthrough the surface back to a detector 30 determines the depth of thesegment sensed by the subject submarine detection system. By steppingdelay 28, one can sample various segments of water column 20 over anumber of sequential pulse pairs so that upon the detection of asubmarine its depth can be ascertained. Note that, for instance, for 10quick sequential pulse pairs one can subdivide the water column into 10depth segments before moving on to search an adjacent water volume.

The output of detector 30 is coupled through a sequential switchingmechanism 32 to provide sequential speckle patterns 34 and 36, withswitch 32 switching between its two switch points 38 and 40 in a timedsequence dependent upon the expected arrival of the pulse returns fromthe double pulse laser. Since the timing for switch 32 is coupled to theoutput of delay circuit 28, the speckle patterns that are detected areonly those which are within a given segment of the water column.

The sequential speckle patterns are cross-correlated on a pixel-by-pixelbasis as indicated at 41 to detect movement between the speckles of thetwo successive speckle patterns and thus provide an overallcross-correlation value 42, the intensity of which indicates the degreeof correlation or decorrelation indicating intensity. Thecross-correlation curve is at a maximum when there is maximum particlemovement between one speckle pattern and the next sequential specklepattern indicating maximum decorrelation as would be expected with rapidparticle movement due to a submarine wake.

As illustrated at 44, one can graphically depict an incident in which asubmarine has been detected at the particular depth at which it isdetected such that with a rise in curve 42 above a predeterminedthreshold 46, one can determine first that that series of returns inwater column 20 was from a submarine wake, and second the depth 44 ofthe moving particles.

Since the sub-hunting apparatus 10 may be airborne, the location anddirection of the output of laser 12 is known. This permits performing asearch pattern by overflying a given area which is many more orders ofmagnitude distance than that coverable by submarine provided with theaforementioned Leonard et al. scheme.

As a result and knowing the X-Y coordinates of the laser beam spot onthe surface of the ocean, one can develop a submarine track 46 in termsof hits or detections at various sequential locations. This means thatthe direction of travel of a submarine can be obtained.

Moreover, because the particulate motion is at a maximum close to thewheel of propeller 48 of the submarine, the degree of decorrelation islargest closest to the submarine. Thus, estimates can be made as to thecloseness of the submarine to the particular water column interrogatedin terms of the intensity of the decorrelation, i.e., the height ofcurve 42. This too leads to a determination of the direction of adetected submarine.

Additionally, the cross-correlation itself may be used to detect thedirection of travel of the submarine. If over time the result of thecross-correlation is weaker, the rate of change of the sequentialspeckle patterns is slower in the search direction, this means that thesubmarine is moving in a direction opposite to the search direction.Likewise, if the correlation is stronger over time in the searchdirection, the search direction is in the direction of travel of thesubmarine.

Referring to FIG. 2, what is illustrated is a number of particles 50 inmotion when the wake envelope 52 is encountered. The particle movementis due to the wake turbulence, whereas particles 54 above and below wakeenvelope 52 are not in motion and are more or less stationary. Note thatthe period of Brownian motion which causes particles in a fluid to moveis much, much larger than the period of the motion due to particlesreacting to a wake.

The result is that it is possible with the subject system to detectmoving particles by sequential double snapshot techniques and thecross-correlation of the results to reliably detect and to locate asubsurface vessel.

More specifically and referring now to FIG. 3, the subject techniquedetects turbulence originating from submarines operating in open oceanand littoral waters. The technique is based upon scattering fromsuspended particles present in all types of ocean water, with particleslocated at random positions. Backscattered laser light interferes toproduce a speckle pattern as illustrated by the two speckle patterns 60and 62, with speckle pattern variation over a short period of timeindicating the presence of moving particles and thus the presence of asubsurface vessel creating a wake.

In FIG. 3, double pulse laser 26 produces the aforementioned two laserpulses 64 and 66 which define and illuminate a range gated water column70 which forms the sample volume for the measurement. A scanner array 72detects returns from the reflected laser radiation, with a change in thesequentially-derived speckle patterns indicating moving particles. Notethat the range gating not only defines sample volume 70 as illustratedat 74, the range gating also indicates the depth of the turbulence andthus the depth of the submarine.

Thus, the backscattered light interferes to produce speckled patterns,with the subject system measuring changes in the speckle patternsproduced by two successive laser pulses with a small but typically 10 to100 microsecond pulse separation time.

Since phase information is converted into intensity information thesubject method can be classified as a coherent LADAR, but not in theclassical heterodyne/homodyne sense where backscattered light isinterfered with a temporally and spatially coherent reference source orlocal oscillator.

In operation, pulsed coherent illumination of the water producesbackscatter from random particle distribution, which, in turn, producesspeckle. Cross correlation of speckle pattern is a measure of changeover time. The sensor is range gated for depth discrimination. In oneembodiment returns are imaged on intensified CCD or APD array with thetwo successive laser pulses captured either by a single fast readoutarray or a dual pixel-registered detector array. Cross-correlation ofthe successive speckle patterns determines the magnitude of specklepattern change. Here, change is manifested as speckle pattern timevariation where the water motion causes the change in speckle return.The phase and resulting intensity variations are due to relativeparticulate motion from an object moving through the water, with phaseinformation converted to measurable intensity information related toparticle motion.

Assuming a dispersed particle concentration that backscatters laserlight, the signal will be dependant upon particle number density.Particles tend to remain relatively still unless disturbed. Hence thereis a correspondingly long time constant effect from Brownian motion,solutions, and the like. If differential motion is present, speckle isdecorrelated and the two temporal waveforms will be different. Thuscorrelation is low. If differential motion is absent speckle iscorrelated and the two temporal waveforms will be identical; thus thecorrelation is high.

It will be appreciated that this method permits detection of a submarinebased upon its wake turbulence. Hence, it is possible to determineposition and heading by sampling the water and following a pattern ofincreasing turbulence. Also, the sensor can be mounted in an aircraftand flown rapidly while scanning the ocean, thus providing significantlygreater coverage than traditional acoustic detection methods. Since thistechnique does not rely upon direct echo reflection, submarines can bedetected hours after they pass through a region, hence facilitatingtracking.

While the present invention has been described in connection with thepreferred embodiments of the various figures, it is to be understoodthat other similar embodiments may be used or modifications andadditions may be made to the described embodiment for performing thesame function of the present invention without deviating therefrom.Therefore, the present invention should not be limited to any singleembodiment, but rather construed in breadth and scope in accordance withthe recitation of the appended claims.

1. A method for detecting, tracking and locating submarines in water,comprising the steps of: projecting monochromatic coherent radiationinto a water column; detecting reflections from particles in the watercolumn to generate speckle patterns; and, deducing from a change in thespeckle patterns the presence of a submarine.
 2. The method of claim 1,wherein the monochromatic radiation is generated by a double pulselaser.
 3. The method of claim 2, wherein the pulses from the doublepulse laser are separated by 10-100 microseconds.
 4. The method of claim1, wherein the speckle patterns are detected using an array ofdetectors.
 5. The method of claim 1, wherein the speckle patterns aredetected using an array of intensified detectors.
 6. The method of claim1, wherein the speckle patterns are detected by an array of detectorsselected from the group consisting of CCD and APD arrays.
 7. The methodof claim 2, and further including the step of range gating the pulsesfrom the double pulse laser so as to determine the depth of the detectedsubmarine.
 8. The method of claim 1, and further including the step ofcross-correlating the speckle patterns.
 9. The method of claim 2,wherein successive laser pulses produce separate speckle patterns andfurther including the step of utilizing dual pixel registered camerasfor the detection of the two speckle patterns.
 10. The method of claim2, wherein successive laser pulses produce separate speckle patterns andwherein the detecting step includes the step of utilizing a fast readoutarray for detecting returns from the two laser pulses.
 11. The method ofclaim 1, and further including the step of deducing from a change inspeckle patterns the direction of travel of a submarine.
 12. The methodof claim 1, and further including the step of deducing the direction oftravel of a submarine from successive detections of the submarine atsuccessive locations.
 13. The method of claim 1, and further includingthe step of deducing the direction of travel of a submarine from thelevel of decorrelation of the speckle patterns.
 14. Apparatus for thedetection of a subsurface vessel, comprising a double pulse laserlocated above the sea surface and generating a collinated double pulseoutput; laser beam direction-determining optics for projecting thecollinated double pulse output of said laser through the air/seainterface to illuminate a water column; at least one detector array fordetecting sub-sea returns from particles in said water column so as toproduce sequential speckle patterns corresponding to the arrival of thesequential pulses reflected from said particles; and, a cross-correlatorcoupled to said sequential speckle patterns for cross-correlating saidspeckle patterns to generate a signal indicative of the degree ofdecorrelation of said successive speckle patterns, whereby decorrelationabove a predetermined threshold is indicative of a subsurface vesselmoving through the water.
 15. The apparatus of claim 14, and furtherincluding a range gate coupled to said detector array for activatingsaid array at a predetermined delay time with respect to the generationof said double pulse output, said predetermined delay determining whatsegment of said water column is probed and the depth thereof.
 16. Theapparatus of claim 15, wherein said range gate is set to excludereflected returns from the air/sea interface.
 17. The apparatus of claim14, wherein said double pulse laser includes a Nd:YAG laser.
 18. Theapparatus of claim 14, wherein the spacing between said double pulses isbetween 10 and 100 microseconds.
 19. A method for ascertaining thepresence of a submersed vessel moving through a body of water,comprising the steps of: illuminating the surface of the water with theoutput of a double pulsed laser; detecting returns from particles in thewater caused to move by the wake produced by the submersed vessel toform a speckle pattern for each returned pulse; cross-correlatingsuccessive speckle patterns produced from returns from successive laserpulses; and, determining from the average decorrelation of the specklepatterns the presence of the submersed vessel.
 20. The method of claim19, and further including the step of determining the direction oftravel of the submersed vessel from the cross-correlation step.
 21. Themethod of claim 19, and further including the steps of flying the doublepulse laser in a predetermined pattern so as to successively illuminatedifferent portions of the surface of the water at known locations, anddetermining from the locations at which the subsurface vessel isdetected, the track of the submersed vessel.
 22. The method of claim 21,and further including the step of determining how far the submersedvessel is from the illuminated portion by the level of decorrelationdetected.
 23. The method of claim 19, and further including range gatingthe returns to detect the depth of the wake produced by the submersedvessel.